148 



THE EMBRYOLOGICAL CRITERION 



(pp. 134-138, 1837). In .Amphibia generally there were no 

 definite lines of separation on the skull itself. "At this 

 stage," he writes of the cartilaginous cranium of the frog, 

 " we find no trace of a veritable division into vertebrae in the 

 cartilaginous trough formed by the twsis cranii and the side 



parts. On the contrary, it is 

 quite continuous, as it is also 

 in the higher Vertebrates during 

 the process of chondrification " 

 (p. 44, 1838). The vertebrae 

 in the membranous or carti- 

 laginous skull could be de- 

 limited in Amphibia by the 

 help of the eye and the ear- 

 labyrinth, which lie more or 

 less between the first and 

 second, and the second and 

 third vertebra;, but, above all, 

 by the vesicles of the brain. 



As in the higher Verte- 

 brates, the visceral arches are 

 associated with the cranial ver- 

 tebrae as their ventral exten- 

 sions, being equivalent to the 

 visceral plates which form the 



ventral portion of the "primitive vertebnu " or primitive 

 segments of the trunk. 



If the three cranial vertebra are not very distinct in the 

 early stages of development when the skull is still membranous 

 or cartilaginous, they become clearly delimited when ossifica- 

 tion sets in. Three rings of bone forming three more or less 

 complete vertebra; are the final result of ossification. 

 The composition of these rings is as follows : 



FIG. 10. Cnuiial Vertebrae and 

 Visceral Arches in Embryo of 

 Pij*. Ventral Aspect. (After 

 Reicheit.) 



